Holographic storage memory systems operate by combining a data-encoded object (or data) beam with a reference beam to create an interference pattern inside a photosensitive storage medium. The interference pattern induces material alterations in the storage medium that generate a hologram. The data is read by sending the same reference beam used to record the hologram into the storage medium. The storage medium will diffract the reference beam reconstructing the stored object beam, which may then be captured by an imager (typically an array of photosensitive elements) and converted to an electronic data signal.
A schematic diagram of a conventional holographic data storage (HDS) system is shown in FIGS. 1A and 1B. FIG. 1A illustrates the HDS system recording data, and FIG. 1B illustrates reading or recalling data.
Referring to FIG. 1A, during the recording process, the reference beam 102 and the object (or data) beam 104 interfere inside the recording volume of a holographic storage medium 106. The two beams originate from the same laser (not shown) and are coherently related. The data to be stored is imprinted in a transverse spatial extent of the object beam 104 by means of a spatial light modulator (SLM 108). Conventional HDS systems employ amplitude modulators—usually binary state only in which the modulator is either ON or OFF. The reference beam 102 is, in the simplest case, a plane wave with specific incident angle. The complex interference pattern between the reference and object beams induces a permanent or semi-permanent index variation in the holographic storage medium 106. Typically, 106-108 bits of binary data can be stored in a single holographic image or hologram, commonly referred to as a page. Multiple hologram pages can be stored in the same physical volume of the holographic storage medium 106 by multiplexing (e.g. angle multiplexing as depicted in FIG. 1A).
Referring to FIG. 1B, to recall a particular hologram page, the same reference beam 102 used to record a particular hologram page is sent to through the holographic storage medium 106. The particular page will diffract the reference beam 102, reconstructing the object beam 104 as encoded with stored data, an imager 110, such as an array of charge—coupled devices (CCD), or a CMOS array captures the reconstructed object image and converts it to electronic data.
Conventional HDS systems all suffer from one or more drawbacks or disadvantages. Referring to FIGS. 1A and 1B, the most common multiplexing method is angular multiplexing in which each hologram page is recorded indexing the reference beam 102 at angle (θ), which are Bragg discriminated from others. This method requires high precision mechanical indexing of the reference beam 102, typically using a mechanism that is relatively slow and subject to wear.
A further disadvantage of conventional HDS systems is the limit on the amount of data that can be stored in a given holographic medium volume imposed by the resolution of the system. In general, for a given optics (i.e. pupil size), the size of a pixel in the SLM is the smallest resolvable feature that can be stored in the medium. Smaller features may be imaged, and therefore stored, but only with rapidly decreasing contrast, which reduces the reliability of the data storage.
Accordingly, there is a need for a holographic data storage system and a method of using the same to quickly and reliably store multiple pages of data without the need for mechanical indexing of a reference beam and with increased data resolution.
The present invention provides a solution to these and other problems, and offers further advantages over conventional spatial light modulators and holographic data storage systems and methods of operating the same.